Back to EveryPatent.com
United States Patent |
5,141,760
|
Davis
,   et al.
|
August 25, 1992
|
Electric deep fat frying method
Abstract
A deep fat frying system has heating elements controlled by triacs operated
in the zero switching mode by a programmed digital processor. The triacs
are switched, and thus power is delivered to the heating elements, at a
rate substantially faster than the thermal cycle of the heating elements
to create a constant or uniform temperature on the surface of the heating
elements, which avoids temperature fluctuations, particularly excessive
peak temperatures, in the heating elements that scorch fat in contact with
the elements and that cause temperatures of the fat to overshoot a desired
cooking set point temperature.
Inventors:
|
Davis; John R. (Shreveport, LA);
Meister; John A. (Haughton, LA);
Roberts; Randy C. (Shreveport, LA)
|
Assignee:
|
Electric Power Research Institute, Inc. (Palo Alto, CA)
|
Appl. No.:
|
702843 |
Filed:
|
May 20, 1991 |
Current U.S. Class: |
426/233; 426/438 |
Intern'l Class: |
H05B 011/00 |
Field of Search: |
426/231,233,237,438
99/330,331,403
219/494,497,501
|
References Cited
U.S. Patent Documents
3734744 | May., 1973 | Albright | 426/231.
|
4282423 | Aug., 1981 | Volz | 99/330.
|
4913038 | Apr., 1990 | Burkett et al. | 99/331.
|
Primary Examiner: Yeung; George
Attorney, Agent or Firm: Hubbard, Thurman, Tucker & Harris
Parent Case Text
This application is a division of application Ser. No. 483,054, filed Feb.
21, 1990, now U.S. Pat. No. 5,038,676.
Claims
What is claimed is:
1. The method of controlling a cooking system having an electrical heating
element submerged in a cooking oil, and the triac power switching means
for controlling alternating circuit power to the heating element which
switch power at zero voltage during zero voltage crossover comprising duty
cycle modulating power to a heating elements by switching the triac power
switching means at a duty cycle rate substantially less than the thermal
time constant of the heating element to maintain a substantially constant
temperature at the surface of the heating element in contact with the
cooking oil.
2. The method of claim 1 wherein the power is duty cycle modulated at a
constant fractional power rate substantially less then maximum power from
initial start up until substantially the set point cooking temperature is
reached.
3. The method of claims 1 or 2 wherein the power is duty cycle modulated to
maintain the cooking oil at a set point temperature without material
overshoot during cooking cycles by proportionally modulating the power in
a preselected temperature band below the setpoint temperature.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to commercial deep fat fryers, and more
particularly relates to computer controlled, high powered, electrically
heated deep fat frying systems utilizing solid state switching devices.
Current deep fat fryers used by commercial establishments for cooking large
quantities of french fries and similar products are typically powered by
three phase 240 volts. All such systems use mechanical or mercury type
contactors to control the high current to the heating elements, often in a
duty cycle modulated manner for temperature control. Each heating element
usually includes a first set of contactors controlled by a high limit
thermostat and associated circuitry which is connected in series with the
primary contactors and the heating elements. The primary contactors are
switched on and off by a thermostat control circuitry in order to achieve
the precise temperature control required to produce consistent food
products of acceptable quality. In this procedure, the primary contactors
must be cycled on and off repeatedly. But, in order to keep the total
switching cycles over the expected lifetime of the contactors to a
reasonably low number, the contactors can only be switched at a relatively
low rate. Duty cycles on the order of 30 seconds are typical. Such duty
cycle periods are often greater than the thermal inertia of the heating
elements resulting in temperature variations at the surface of the
elements in contact with the cooking oil.
The use of triacs, a semiconductor switching device, as the main switching
device for controlling power to a heating element has been suggested by
prior art devices as disclosed in U.S. Pat. No. 3,946,200, to Juodikis,
issued Mar. 23, 1976. This patent discloses the use of a triac controlled
by a bridge circuit and zero voltage switches to provide switching of the
triac at zero cross over points to achieve proportional control of the
power to the heating element and thus a constant temperature.
Commercial deep fat frying units of a type used in fast food establishments
for cooking large loads of french fries and the like, typically employ a
pair of separate cooking vats within each unit, and may have a number of
different units. In these systems, the cooking oil degrades rapidly when
subjected to excessive temperatures, and the cooking oil, together with
power, are among the higher cost items of the business. In the normal
operation of previous systems, the relatively long duration of the duty
cycles result in the elements being left on for a number of seconds,
followed by an off cycle during which the element cools down. As a result,
the oil is exposed to peak temperatures greater than would be necessary if
a more uniform temperature of the heating elements could be maintained.
These types of cooking systems have in recent times employed computers to
control the on/off cycle of the cooking elements in a manner to minimize
the maximum temperature of the heating elements during the melt cycle when
solid shortening is first being melted and may not be in contact with
portions of the heating elements. Even if liquid shortening is used,
during the morning heat up cycle, the oil must be preheated to the set
point or other standby temperature. The oil is typically heated rapidly
with full power since there are typically a number of separate frypots,
the power consumption of the facility exceeds the peak power defined by
the local utility which often results in increased rates per unit of
power.
SUMMARY OF THE INVENTION
The present invention is concerned with an improved deep fat frying system
which utilizes heating elements, each controlled by triacs operated in the
zero voltage switching mode by a programmed digital processor. The triacs
are switched "on" and "off" at duty cycle rates substantially faster than
thermal cycle rate of the heating elements so that a uniform desired
temperature is obtained. Duty cycle rates of less than a second are
typical, with switching lower to power cycle rates available. The
resulting proportional control, together with the programmed digital
processor, allows the fryer to be controlled in all modes of operation to
advantageously reduce peak power demand at various times during the daily
operating cycle, minimize maximum temperature at any time required for
efficient heat transfer to the oil for the given operating requirement,
improved control of the temperature of the cooking oil during the cooking
cycle, and especially reduce temperature overshoot.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects are accomplished in accordance with a preferred
embodiment of the present invention as illustrated in the following
drawings and detailed description wherein:
FIG. 1 is a perspective view of a multivat frying apparatus in accordance
with the present invention;
FIG. 2 is a schematic circuit diagram of the electrical system of the
frying apparatus of FIG. 1; and
FIGS. 3, 4, 5 and 6 are flow diagrams illustrating how the digital
processor of device of FIGS. 1 and 2 is programmed to achieve certain
control functions.
DETAILED DESCRIPTION OF THE DRAWINGS
Referring now to the drawings, and in particular to FIG. 1, a system in
accordance with the present invention is indicated generally by the
reference numeral 10. The system 10 includes a pair of cooking vats 12a
and 12b disposed in side-by-side relationship with a single floor
supported frame. The vats 12a and 12b include a three phase resistance
type electrical heating element 14a and 14b, respectively, which are
immersed within the cooking medium which may be any suitable shortening or
oil. A pair of temperature sensing devices 16a and 18a and 16b and 18b for
sensing the actual temperature of the cooking oil in each vat. The devices
16a and 16b are typically merely resistive temperature devices (RTD) used
to sense temperatures and feed such information to the digital controller
which will presently be described, while the thermostats 18a and 18b are
mechanical high limit devices which are normally closed and open when a
selected high limit temperature is reached. A single digital computer with
appropriate single or multiple read outs as represented by the control
panel 20 is used on combination with other circuit means to control the
heating elements of the two cooking vats 12a and 12b, as will hereafter be
described in greater detail. Referring now to FIG. 2, the power supply
comes in on lines L1, L2 and L3 and is typically 240 volts, three phase
alternating current. FIG. 2 depicts identical circuits for the left and
right frypots 12a and 12b. Accordingly, the reference numerals used to
identify the identical components for the left and right frypots will be
designated by the same reference numerals followed by the reference
characters "a" for the left-hand vat and "b" for the right-hand vat. For
convenience, only the reference numerals and characters for the left-hand
vat will now be described, it being understood that the control circuit
for the right-hand vat is identical to and functions in an identical
manner to that of the left-hand vat, both under the control of the digital
computer 20. The power is thus applied through the three contactor
switches of a mechanical relay indicated generally by the reference
numeral 24a and includes three switches 26a, 28a, and 30a, driven by a
coil 32a. The power from lines L1, L2 and L3 are thus applied to the nodes
34a, 36a and 38a, respectively, of a delta configured power system. A
first heating element 40a is connected in series with a triac 42a and a
contactor 26a, between nodes 34a and 36a. Similarly, a heating element 46a
is connected in series with a triac 48a and contactor 28a between nodes
36a and 38a, and the third heating element 52a is connected in series
with a triac 54a and a contactor 30a between nodes 38a and 34a. Each of
the three power triacs is protected by a capacitor 58a and veristor 60a
connected in parallel with the respective triacs. The triacs 42a, 48a and
54a are switched "on" by commercially available devices 60a62a and 64a.
These devices may be Motorola Opto., Isolators part No. MOC3041. These
devices effect zero voltage switching of the respective triacs on the next
occurrence of zero voltage after a current is applied to the input of the
devices. The contactor 24a is controlled by the high limit mechanical
thermostat 18a which is normally closed until the high limit temperature
is reached. The coil 32a which operates the contactor 24a is powered by a
24 volt power transformer 66a which is switched by a control circuit which
includes a 12 volt power supply 68, a manually actuated on/off switch
represented at 70a . An initial latching circuit is provided by a
capacitor 72a, a resistor 74a, and resistor 86a and contact 84a of a relay
indicated generally by the reference numeral 80a, the contacts 82a and 84a
of which are normally in the position illustrated. Thus when the system is
first turned on by closing the manual switch 70a, power is supplied
through thermostat 18a and capacitor 12a and resistor 74a to close relay
80a. The current is supplied through resistor 86a, switch 84a, and
resistor 74a to latch the relay 80a until power is interrupted by switch
70a or thermostats 18a. When contact 82a is closed, 24 volt power is
applied to the coil 32a of relay 24a to close the power contactors. A pair
of light emitting diodes 90a, together with a resistor 92a are provided in
parallel with the coil 32a to provide an indicator light for trouble
shooting purposes.
The circuit through the normally closed thermostat 18a is also provided to
a diode bridge 94a which directs the current through the triac controller
60a, 62a and 64a. The series circuit is completed through a light emitting
diode 96a provided for trouble shooting and resistor 98a to the bridge
94a. The circuit is then completed through a switching transistor 100a to
ground. The switching transistor 100a is turned "on" by the programmed
digital processor 20 in order to activate the outputs from the controller
60a, 62a, and 64a as will presently be described.
In the event the high limit thermostat 18a opens as a result of a high
temperature limit being exceeded, current to the coil 76a through resistor
86a, switch contact 84a, and resistor 74a is interrupted causing contact
82a to move to the open position illustrated. In that position, current
from the 24 volt power source is supplied through relay 32a, switch
contactor 82a, diode 102a and resistors 104a and 106a to ground. A voltage
signal is thus provided to the digital processor 20 through line 108a
which is used to provide high temperature alarm for the operator.
In the operation of a system of FIG. 2, the switch 70a is manually closed
to instantaneously connect power through capacitor 72a and resistor 74a to
energize coil 76a. Relay contactors 82a and 84a are then closed to provide
holding current to the coil 76a through resistors 86a, contact 84a, and
resistor 74a to the coil 26a and ground. This provides 24 volt power by
way of closed contact 82a to coil 32a to close the mechanical contactors
26a, 28a and 30a to provide line power to the nodes 34a, 36a and 38a.
When the switch 70a is closed, power is also provided through high limit
thermostat 18a, and diode bridge 94a to the optical triac controllers 60a,
62a and 64a, diodes 96a, resistor 98a, and the other half of the bridge
94a to the collector of transistor 100a. Thus, the system is ready for
operational control by the digital processor 20. The digital processor
provides a voltage to the transistor 100a which turns on the triac
controllers 60a, 62a and 64a whenever power is required. The triac
controllers then turn on the respective triacs 42a, 48a and 54a at the
next zero voltage crossing of the AC power signal imposed across the
respective triac. The capacitors 58a and veristors 60a protect the triacs
from line surges during this switching. Thus, the triacs can be switched
on and off at each zero crossing, if desired, by the digital processor.
In the event the temperature of the cooking oil in the vat exceeds the high
limit temperature setting as sensed by the high limit thermostat 18a, the
thermostat opens. This immediately disables any current from the optical
controllers 60a62a and 64a so that the triacs 42a, 48a and 54a cannot be
turned on, thus immediately and directly assuring that no further current
is supplied through these devices. More importantly, the opening of
contact 82a also causes the relay 24a to fall out, thus opening the
mechanical contactors 26a, 28a and 30a, thus disconnecting all power from
the heating elements. This provides a mechanical fail safe system in the
event the solid state devices in series with the heating elements should
short out and be the cause of the overheat condition. At the same time,
the switch 82a provides current from the 24 volt power supply through the
coil 32a, diode 102a and the voltage divider formed by resistors 104a and
106a, thus providing a voltage signal on line 108a to the digital
processor which is used to provide the appropriate alarm to the operator
that a high limit condition has occurred.
The digital controller 20 controls the transistors 100a and 100b to control
the temperature of the cooking oil in the frypots 12a and 12b,
respectively, utilizing the program subroutines illustrated in FIGS. 3, 4,
5 and 6. In general, the power to the heating elements is switched "on"
and "off" by the triacs based for different portions of the total 500
millisecond duty cycle period to provide modulation. This is achieved by
using a decrementing counter which can be set from zero to 250 counts,
incrementing at 2 millisecond intervals. Thus, a full duty cycle
represents 30 cycles of a 60 Hz power supply. Accordingly, the triacs can
be turned "on" at half waveform intervals for from zero to thirty sinewave
cycles. Because of this very short duty cycle, the power to the heating
elements is effectively variable over the entire rage from full "off"to
full "on", particularly when viewed in the context of the thermal inertia
of the heating elements.
In normal operation, the digital processor includes data input means for
inputting a large variety of data including the idealized temperature at
which a particular food product is to be cooked, which is referred to as
the "set point" temperature. An idealized cooking time is also provided
which is the time required if the set point temperature will be maintained
at all times during the cooking period. In some cases, the set point
temperature may vary over the cooking period. In practice, it is
impossible to control the temperature of the cooking oil precisely at the
set point temperature, even if substantial excess heating capacity is
provided by the rated size of the heating elements. Thus, the temperature
may vary significantly depending on the cooking load, and accordingly,
time adjustments are typically provided by the digital processor based on
the actual temperature of the oil during the cooking cycle. In order to
achieve a rapid recovery to the desired set point temperature after a
heavy cooking load has been introduced and cooled the oil, it is desirable
to provide high heat power to provide rapid recovery. However, the more
rapid the recovery capability, the more likely the temperature will
overshoot the desired set point temperature after the heating is turned
off. Any significant overshoot is unacceptable because it significantly
degrades the quality of the cooked product. As a result, the digital
processors have heretofore estimated when the power should be turned "off"
to achieve the desired set point within the minimum recovery time yet with
the minimum overshoot.
The present invention provides for the first time a practical approach to
providing a proportional control of the heat applied to the heaters
related to the difference in the actual temperature and the desired set
point temperature. Thus, in the present invention, a proportional control
band equal to ten degrees below the set point temperature is provided and
the power applied to the heating elements is proportional by the duty
cycle modulation to provide a proportional relationship of maximum power
to the difference in the actual temperature and the set point temperature.
By substantially oversizing the heating elements and available power, the
recovery time to set point temperature can be very rapid, with minimum
overshoot because of the precise control.
Additionally, during the melting of solid shortening placed in the
container for the first time, and in the instance of the initial warm-up
at the start of business each day, it is desirable to provide a special
procedure to ensure that the elements are not turned full on below the
start of the proportional band, which is selected as ten degrees below the
set point. During this period, the power is arbitrarily maintained at some
fractional level, in the present case about twenty percent until either
the set point temperature is achieved, or optionally, the corresponding
twenty percent point within the proportional control band.
Referring now to FIG. 3, the power subroutine is periodically entered as
time permits during routine operation of the digital processor. The clock
interrupt subroutine illustrated in FIG. 4 is entered every two
milliseconds and is used to control the duty cycle time and to generate
the command to turn the respective transistors 100a and 100b on and
thereby provide heat to the respective frypots 12a and 12b. The
subroutines of FIG. 5 for calculating the "on" time and of FIG. 6 for
calculating the "off" time are actually part of the power subroutine of
FIG. 3. As previously mentioned, during start-up, a melt cycle mode is
automatically entered into by the digital controller, or on command from
the operator.
At two millisecond intervals, the clock interrupt subroutine of FIG. 4 is
started. The clock interrupt subroutine first checks to see if the time in
the decrement counter is equal to zero as represented by decision block
120. If the time is not zero, the time counter is decremented by one count
as represented by the decision block 122 and the interrupt subroutine is
exited. On the other hand, if the time is equal to zero, the on/off flag
is checked as represented by decision block 124. The on/off flag indicates
whether the counter is timing the "on" power portion of the duty cycle or
the "off" power portion. If the on/off flag indicates that the power on
duty cycle was being timed and the timer is now zero, the program then
proceeds to decision block 126 where the saved off time is checked to see
if it is zero. If zero, then there is not desire to proceed to a power off
cycle, bu the power on/off flag is flipped as represented by block 128 in
preparation for the next clock interrupt subroutine. If the saved power
off time is not equal zero, the power is turned off as represented by
block 130, and the calculated power off time is set in the timer as
represented in block 132 before proceeding to flip the power on/off flag
by block 128 to indicate that a power off cycle is being timed.
On the other hand, if the decision block 124 indicates that the flag is in
the power off condition, the saved power on time is checked as represented
by decision block 134. If the saved power on time is equal to zero, the
power on/off flag is flipped as represented by block 128 in preparation
for the next interrupt subroutine. On the other hand, if there is time in
the power on time location, the power is turned on as represented by block
136, and the calculated power on time is set in the timer as represented
by block 138 before flipping the power on/off flag as represented by block
128 to set up a power on duty cycle and exiting the program.
The power subroutine illustrated in FIG. 3 basically determines whether to
operate the system in the melt cycle mode or in the proportional control
mode. Upon entering the subroutine, the melt cycle flag is first checked
as represented by decision block 150. If the melt cycle flag is set, the
subroutine proceeds to get the temperature differential between the set
point temperature and the actual temperature as represented by block 152.
This temperature differential is then checked to see if it indicates that
the actual temperature is greater than or equal to the set point
temperature. If the temperature differential indicates that the actual
temperature is less than the set point temperature, it is desired to stay
in the melt cycle mode, the melt cycle on time of 50 ticks and off time of
200 ticks is saved for use by the intercept subroutine of FIG. 4 as
represented by block 155, and the power subroutine is exited.
On the other hand, if the temperature now exceeds the set point
temperature, the program proceeds to clear the melt cycle flag as
indicated by block 156, and again fetches the temperature differential as
represented by block 158, where it is now checked to determine if the
temperature differential indicates that the cooking oil temperature is
greater than the set point temperature as represented by decision block
160. If greater, the power on time is cleared, or set to zero, and the
power off time is set to 40 milliseconds as represented by block 162 so
that the power will stay off for 20 counts before again entering the
subroutine, to allow the temperature to cool down.
If the temperature differential is not greater than the set point, the
temperature is then checked to see if it is less than or equal to 10
degrees, i.e., to see if the temperature is in the ten degree proportional
control band. If the temperature differential is greater than 10 degrees,
the one time is set to the maximum ticks as represented by block 164
before exiting the program so that the power will stay on for the entire
next duty cycle of 0.5 seconds. If the temperature differential is less
than or equal to 10 degrees, the power subroutine then proceeds to
calculate and save the power on time as represented by block 168, then
calculates and saves the power off time as represented by block 170 before
exiting the routine.
The power on time is calculated as represented by the subroutine in FIG. 5
where the temperature differential is loaded as represented by block 172,
the differential is multiplied by 25 as represented by block 174, and the
resulting number of power on ticks is saved as represented by block 176.
These ticks will subsequently be inserted into the timer by the interrupt
subroutine of FIG. 4. Similarly, the power off time is calculated by the
subroutine illustrated in FIG. 6 where the maximum number of ticks is
loaded, as represented by block 172. In the present case, the duty cycle
is 250 ticks, which is 500 milliseconds, and therefore 250 is loaded at
block 172. Next, the one ticks, calculated and saved in block 176, are
subtracted from the maximum ticks as represented by block 174 to produce
the off ticks which is then saved as represented in block 176.
Thus, it will be seen that an improved cooking system has been provided in
which very high power electrical heating elements, oversized if
economically practical can be used in a deep fat fryer and operated in a
manner to minimize the maximum surface temperatures of the heating element
surfaces during all phases of operation, and thus minimize degradation of
the cooking oil. Yet, the precise proportional control of high power
devices provide a means by which recovery time after a heavy cooking load
such as frozen french fries have been suddenly dumped in the oil can be
maximized, while minimizing overshoot at the set point temperature. A
method of operation has also been disclosed by which the maximum power
consumption of a battery of two or more of the units can be minimized by
maintaining a low power heat up during melt cycle or initial warm-up all
the way to the set point temperature. This is all achieved while also
providing longer life because of the solid state switching devices, and
while simultaneously preserving the fail safe features of the mechanical
thermostat and mechanical contactors in high limit conditions.
Although preferred embodiments of the invention have been described in
detail, it is to be understood that various changes, substitutions and
alterations can be made therein without departing from the spirit and
scope of the claims as defined by the appended claims.
Top